The present invention relates to an optical fiber bundle.
A bundle of optical fibers having a small diameter of about 0.01 to 0.02 mm is mainly used in an endoscope used for examining the insides of organs such as the stomach, intestines and bronchial tubes. As the tip of the endoscope is guided in a desired direction, the optical fibers are subjected to cyclic bending at small radii of curvature and become highly prone to breakage. Therefore, it has been long desired in the fiber optics industry to develop a fiber bundle that will not easily break upon repeated microbending.
The practice conventionally adopted to meet this need is to apply a coating of an antifriction agent such as a molybdenum disulfide powder or an oil on all parts of the optical fibers, except at both ends where the fibers are bound together, so that the individual fibers will slide smoothly against one another. The fibers treated in this manner are covered with a flexible tube.
Optical fibers themselves neither expand nor contract, and hence when an optical fiber bundle is bent at a small radius of curvature, all fibers, in an attempt at converging in the neutral plane, will spread laterally in the flexible tube, causing the latter to deform into a flat shape resembling a cobra's hood, as shown in FIG. 3. Being coated with an antifriction agent to provide better slippage between individual optical fibers, the prior art optical fiber bundle permits all fibers to move freely and individually. Therefore, if the bundle is subjected to repeated deformation on account of cyclic bending at small radii of curvature, the fibers will become twisted or entangled, bringing them out of alignment and breaking at portions where misalignment occurs.
If optical fibers in an image transmitting fiber bundle break, a black spot will appear on the viewing screen, making accurate diagnosis difficult. Moreover, if fiber breakage occurs in an illuminating fiber bundle, the quantity of illuminating light is decreased, reducing the brightness of the screen. In this case too, diagnosis becomes more difficult.
Therefore, if the prior art optical fiber bundle is to be employed in an endoscope, the inner space of the flexible pipe attached to the endoscope and which is to be inserted into an organ of interest is made large enough to ensure that the fiber bundle will not break upon compression by a forceps channel, air/water tubes, or any other element that is to be assembled in the flexible pipe. However, this prevents the insertion of many elements or thick elements into the flexible pipe, thereby putting considerable limits on the capabilities of the endoscope.
An object, therefore, of the present invention is to provide a durable optical fiber bundle that is free from the aforementioned problems of the prior art and which can be subjected to cyclic bending at a small radius of curvature without causing fiber breakage.
Further, the present invention relates to an improvement of the flexible tube serving as an outer sheath for the optical fiber bundle.
Conventional image transmitting or illuminating optical fiber bundles for use in endoscope are commonly sheathed with a flexible tube made of silicone rubber before they are slipped into the inserting portion of an endoscope. However, if the optical fiber bundle is cyclically bent in such areas at the curving area of the inserting portion, the fiber bundle is compressed by the forceps channel and other elements in the inserting portion of the endoscope, which can be a cause of damage to the optical fibers in the bundle. In order to increase the durability of the optical fibers, it has been proposed that the flexible tube of silicone rubber be reinforced with an outer tube. (See, for example, Japanese Unexamined Published Utility Model Application No. 74110/1985).
However, a dual sheath on the optical fiber bundle not only increases the production cost of the fiber bundle, but it also increases the diameter of the fiber bundle and the forceps channel and other elements to be incorporated in the inserting portion of an endoscope must be made thinner by a corresponding amount, leading to a reduction of the capabilities of the endoscope.
Another object of the present invention therefore is to eliminate the drawbacks of the prior art and to provide a durable optical fiber bundle for an endoscope that is sheathed with a single outer tube and which yet can be bent cyclically without causing breakage of the optical fibers.